CitMHS family explained

The Citrate-Mg²⁺:H⁺ (CitM) / Citrate-Ca²⁺:H⁺ (CitH) Symporter (CitMHS) Family (TC# 2.A.11) is a family of transport proteins belonging to the Ion transporter superfamily.^[1] Members of this family are found in Gram-positive and Gram-negative bacteria, archaea and possibly eukaryotes. These proteins all probably arose by an internal gene duplication event. Lensbouer & Doyle (2010) have reviewed these systems, classifying the porters with three superfamilies, according to ion-preference:^[2]

1) Mg²⁺-preferring,

2) Ca²⁺-preferring, and

3) Fe²⁺-preferring.

A representative list of proteins belonging to the CitMHS family can be found in the Transporter Classification Database.^[3]

CitM and CitH

Two of the characterized members of the CitMHS family, both citrate uptake permeases from Bacillus subtilis, are CitM (TC# 2.A.11.1.1) and CitH (TC# 2.A.11.1.2).

Function

CitM is believed to transport a citrate²⁻-Mg²⁺complex in symport with one H⁺ per Mg²⁺-citrate while CitH apparently transports a citrate²⁻-Ca²⁺ complex in symport with protons.^{[4] [5]} The cation specificity of CitM is: Mg²⁺, Mn²⁺, Ba²⁺, Ni²⁺, Co²⁺, Ca²⁺ and Zn²⁺, in this preferential order. CitM is highly specific for citrate and D-isocitrate and does not transport other di- and tri-carboxylates including succinate, L-isocitrate, cis-aconitate and tricarballylate.^{[6] [7]} For CitH, the cation specificity (in order of preference) is: Ca²⁺, Ba²⁺ and Sr²⁺. The two proteins are 60% identical, contain about 400 amino acyl residues and possess twelve putative transmembrane spanners. A CitM homologue in S. mutans transports citrate conjugated to Fe²⁺ or Mn²⁺ but not Ca²⁺, Mg²⁺ or Ni²⁺.^[8]

The transport reactions catalyzed by (1) CitM and (2) CitH, respectively, are:

(1) Citrate • Mg (out) + nH⁺ (out) ⇌ Citrate • Mg (in) + nH⁺ (in)

(2) Citrate (out) + nH⁺ (out) ⇌ Citrate (in) + nH⁺ (in)

(3) Citrate • Ca²⁺ (out) + nH⁺ (out) ⇌ Citrate • Ca²⁺ (in) + nH⁺ (in)

See also

• Membrane transport protein
• Carrier protein
• Calcium in biology
• Transporter Classification Database

Notes and References

1. Prakash. Shraddha. Cooper. Garret. Singhi. Soumya. Saier. Milton H.. 2003-12-03. The ion transporter superfamily. Biochimica et Biophysica Acta (BBA) - Biomembranes. 1618. 1. 79–92. 0006-3002. 14643936. 10.1016/j.bbamem.2003.10.010.
2. Lensbouer. Joshua J.. Patel. Ami. Sirianni. Joseph P.. Doyle. Robert P.. 2008-08-01. Functional characterization and metal ion specificity of the metal-citrate complex transporter from Streptomyces coelicolor. Journal of Bacteriology. 190. 16. 5616–5623. 10.1128/JB.00456-08. 1098-5530. 2519374. 18556792.
3. Web site: 2.A.11. The Citrate-Mg2+:H+ (CitM) Citrate-Ca2+:H+ (CitH) Symporter (CitMHS) Family. Transporter Classification Database. 2016-03-04.
4. Boorsma. A.. van der Rest. M. E.. Lolkema. J. S.. Konings. W. N.. 1996-11-01. Secondary transporters for citrate and the Mg(2+)-citrate complex in Bacillus subtilis are homologous proteins. Journal of Bacteriology. 178. 21. 6216–6222. 0021-9193. 178492. 8892821. 10.1128/jb.178.21.6216-6222.1996.
5. Krom. B. P.. Warner. J. B.. Konings. W. N.. Lolkema. J. S.. 2000-11-01. Complementary metal ion specificity of the metal-citrate transporters CitM and CitH of Bacillus subtilis. Journal of Bacteriology. 182. 22. 6374–6381. 0021-9193. 94783. 11053381. 10.1128/jb.182.22.6374-6381.2000.
6. Li. H.. Pajor. A. M.. 2002-01-01. Functional characterization of CitM, the Mg2+-citrate transporter. The Journal of Membrane Biology. 185. 1. 9–16. 10.1007/s00232-001-0106-1. 0022-2631. 11891560. 8677932.
7. Warner. Jessica B.. Lolkema. Juke S.. 2002-11-01. Growth of Bacillus subtilis on citrate and isocitrate is supported by the Mg2+-citrate transporter CitM. Microbiology. 148. Pt 11. 3405–3412. 10.1099/00221287-148-11-3405. 1350-0872. 12427932. free.
8. Korithoski. Bryan. Krastel. Kirsten. Cvitkovitch. Dennis G.. 2005-07-01. Transport and metabolism of citrate by Streptococcus mutans. Journal of Bacteriology. 187. 13. 4451–4456. 10.1128/JB.187.13.4451-4456.2005. 0021-9193. 1151779. 15968054.